The present invention relates to an improvement in microencapsulated imaging systems, and more particularly, to a method for improving image speed and intensity, while lowering cost.
Encapsulated imaging systems frequently employ diluents in conjunction with the typical color formers and solvents found in the internal phase of microcapsules. Diluents are used because they are less expensive than solvents and they enable the manufacturer to provide an imaging system having acceptable image speed and intensity at a reduced cost. However, there are limits to the amount of diluent that can be encapsulated without adversely affecting the performance of the encapsulated system. High diluent concentrations can create supersaturated solutions of the color former which are prone to crystallization or "drop out" of the color former from solution. The crystallized color former does not efficiently transfer to the co-reactive compound or sheet and, as a result, there is a corresponding decrease in the speed of image development and overall image intensity.
One proposed solution to this problem is disclosed in U.S. Pat. No. 4,684,574 (Pietsch et al.). This patent discloses the precise metering of the two components (color former solution and diluent) into an in-line mixer just prior to emulsification. The basic underlying concept is that by immediately emulsifying the mixture after adding the diluent there is not sufficient time for the color former to crystallize out of solution before emulsification and encapsulation. The resulting microcapsules are claimed to be comparable to those prepared with pure solvent. However, it has been found that crystallization can still occur in microcapsules prepared by this method. At high enough levels of diluent, the color former can crystallize out either in the mixing chamber or in the encapsulated product.
U.S. Pat. No. 4,636,818 to Jerabek discloses a carbonless copying system including two types of microcapsules. The first type of microcapsule contains either a solution or suspension of one of the two mark forming components of the carbonless system. The second type of microcapsule encapsulates only solvent. The solvent-only microcapsules disclosed in Jerabek enhance imaging capability without an increase in usage of dye precursors. However, the additional solvent usage increases the cost of the microcapsules. It would be desirable to achieve the same effect with less expensive diluent.
Accordingly, it is desirable to provide an imaging system wherein a high concentration of diluent can be incorporated into a microencapsulated imaging system to improve speed of image development and image intensity.